The slow access time, of the order of 5-10 ms, for an input/output (IO) transaction performed on a disk has led to the need for a caching system between a host generating the IO transaction and the disk. A cache, a fast access time medium, stores a portion of the data contained in the disk. The IO transaction is first routed to the cache, and if the data required by the transaction exists in the cache, it may be used without accessing the disk.
One goal of an efficient caching system is to achieve a high “hit” ratio, where a high proportion of the data requested by IO transactions already exists in the cache, so that access to the disk is minimized. Other desirable properties of an efficient caching system include scalability, the ability to maintain redundant caches and/or disks, and relatively few overhead management transactions.
U.S. Pat. No. 5,694,576 to Yamamoto, et al., whose disclosure is incorporated herein by reference, describes a method for controlling writing from a cache to a disk by adding record identification information to a write request. The added information enables the cache to decide whether data written to the cache should or should not be written to the disk.
U.S. Pat. No. 6,457,102 to Lambright, et al., whose disclosure is incorporated herein by reference, describes a system for storing data in a cache memory that is divided into a number of separate portions. Exclusive access to each of the portions is provided by software or hardware locks. The system may be used for choosing which data is to be erased from the cache in order to make room for new data.
U.S. Pat. No. 6,434,666 to Takahashi, et al., whose disclosure is incorporated herein by reference, describes a caching system having a plurality of cache memories, and a memory control apparatus that selects the cache memory to be used. The memory control apparatus selects the cache so as to equalize use of the cache memories.
U.S. Pat. No. 6,490,615 to Dias, et al., whose disclosure is incorporated herein by reference, describes a scalable cache having cache nodes for storage servers. On receipt of a read request, the cache nodes serve the request or communicate with each other to cooperatively serve the request.
U.S. Pat. No. 5,666,512 to Nelson, et al., whose disclosure is incorporated herein by reference, describes a data storage system comprising a number of disks which are managed by a memory manager. The memory manager maintains a sufficient quantity of hot spare storage space for reconstructing user data and restoring redundancy in the event that one of the storage disks fails.
U.S. Pat. No. 6,418,068 to Raynham, whose disclosure is incorporated herein by reference, describes a self-healing memory comprising a primary memory cells and a spare memory cell. A detector is able to detect an error in one of the primary memory cells. When an error occurs, a controller maps the memory cell having the error to the spare memory cell.
U.S. Pat. No. 6,449,731 to Frey, Jr., whose disclosure is incorporated herein by reference, describes a method to manage storage of an object in a computer system having more than one management storage process. A memory access request is routed to a first storage management process, which is determined to have failed. The request is then routed to a second storage management process, which implements the request.
U.S. Pat. No. 6,530,036 to Frey, Jr., whose disclosure is incorporated herein by reference, describes a self-healing storage system that uses a proxy storage management process to service memory access requests when a storage management process has failed. The proxy accesses relevant parts of a stored object to service the memory access requests, updating the stored object'information to reflect any changes.
U.S. Pat. No. 6,591,335 to Sade, et al., whose disclosure is incorporated herein by reference, describes a method for managing a cache by providing data from a disk storage area to a first and a second cache memory, where the first and second cache memories contain at least some data that is not stored in the other one of the cache memories. In response to data being modified while stored in the cache memories, the same data is written to both of the cache memories.
U.S. Pat. No. 6,604,171 to Sade, whose disclosure is incorporated herein by reference, describes managing a cache memory by using a first cache memory, copying data from the first cache memory to a second cache memory, and, following copying, using the second cache memory along with the first cache memory.